If something is still making someone money then you are basically asking them to forgo any future earnings and give it away for free.
Personally, I have no problem with programmers keeping the code to themselves. What I would like though is if they have got bored or moved on to something that's actually going to make them money this day and age (like a PC or mobile app) then release their Amiga sources and let someone else take over if such a person is out there.
But ultimately how fruitful will this be? Dopus 4 sources were released and there was maybe 2 updates 5 years ago and nothing since. Scratch that, it was 12 years ago!! (Although looking at the source there have been some changes up to 2012 at least)
I don't know what you wish to gain really.
I really think you over estimate the popularity of the platform for programmers, I imagine the enthusiasts are working on their own stuff already and anyone else has already moved on to current platforms and only uses the Amiga for nostaligia not for scratching his head for hours on end trying to figure out someone elses code.